3D? I don't see it...

There may be some inherent defects in the eye that could hamper more than just vision. Geetha Srinivasan offers some insight into what could possibly be wrong with the all-important organ.

Ganesh returned home with nausea and headache after watching a 3D movie, while his friends enjoyed every bit of the technologically brilliant masterpiece.

Mariamma had to remove those 3D glasses and shut her eyes for 10 minutes before she could resume watching the movie. While Supreetha’s cousins were busy cheering and trying to catch the flowers and goodies moving close to them in the theatre, all she could see was a blur.

Why is it that some people don’t enjoy 3D movies as much as others? To answer that, we first need to understand how these movies work.

The basics

What is 3D vision? “Our right and left eyes see objects from different angles. The right eye sees more objects towards the left, while the left eye sees more towards its right.

These two slightly different images are sent to the brain separately by the right and left eyes. The nearer objects will have more of this difference than farther objects and our brain assesses this minute disparity and tells us how far an object is away from you,” explains Dr M S Ravindra, medical director at Karthik Netralaya, Bengaluru.

This is stereo vision, where an object that is far away appears far, and the one that is near appears near. If you close one eye, you are left with only false cues of three dimensions; it is not accurate. Close one eye and try to bring the index fingers of both your hands closer to each other at arm length’s distance; it is difficult for their tips to meet each other accurately.

Remember, one of our eyes is not a spare or stepny; we need both eyes working well and together to face this challenging world.

3D movies use this capability of our brain to create fantastic effects. There are two movies projected on the screen, with a disparity. The rays are polarised, so that when you wear the 3D glasses, the right eye sees one image, and the left eye sees the other image.

The images are dimmer due to polarisation. If the disparity is more, the brain thinks that the object is closer, if the disparity is less, it mentally rebuilds a farther image. To enjoy the movie, the visions of both eyes should be good and equal, the muscle balance between the two eyes needs to be good and convergence should be adequate, explains Dr Ravindra.

The analysis

Supreetha’s one eye was defective due to Amblyopia. If her parents had taken her to an eye specialist when she was three or four years old, her blindness could have been treated. Amblyopia cannot be treated after the age of nine, although to get good binocular vision, treatment should be completed by the age of five.

It involves wearing glasses and applying a patch to close the good eye so as to exercise the defective eye. Supreetha now has to spend her entire life using one eye. She cannot enjoy 3D movies and 3D comics, and can never become an aircraft pilot either.

Mariamma had anisometropia, where one eye had higher power than the other. Although she sees well with both eyes, her two eyes send images of different sizes to the brain, which has to do extra work to fuse the two images. Brain can usually take up this additional work, but in an artificial scenario like a 3D movie, it becomes too much for it to bear. Today, she has gotten this issue corrected by advanced EpiLasik surgery and no longer has to use unsightly glasses of different powers.

The root cause for Ganesh’s headache was convergence insufficiency. Although his eye muscles are used to computer work, a 3D movie puts extra demand on his convergence. Convergence exercises can do him good to some extent. If the problem persists and creeps into his routine life, he could take the help of prisms or get a minor surgery to correct his large exophoria.

You may have abnormal retinal correspondence or a squint and need assessment of ocular motility disturbances. You may not be aware of this at all until you watch a 3D movie. Phoria is a type of squint where the eyes have a tendency to deviate, but are kept together by binocular efforts and needs.

There is a continuous strain on eye muscles to keep the eyes straight. Rarely, diseases like nerve palsies, diabetes, tumours, trauma, even minor head injuries, congenital diseases, myasthenia, thyroid and the like can be the cause. Some of them are treatable. A squint, although ideally corrected before the age of five, can be corrected at any age.

There is another big 3D movie waiting to hit the theatres. If you’ve been having problems, stop blaming the glasses. It’s probably your eyes. Hurry and visit a good eye doctor.

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